Means compensating for the time delay of a transmission line



E. c. GROGAN 2,923,307

2 Sheets-Sheet 1 Feb. 2, 1960 MEANS COMPENSATING FOR THE TIME DELAY OF ATRANSMISSION LINE Filed Dec. 29, 1955 INVENTOR.

EDWARD c. GROGAN BYi ATTORNEY.

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MEANS COMPENSATING FOR THE TIME DELAY OF A TRANSMISSION LINE Filed D80.29, 1955 FIG. 2

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EDWARD C. GROGAN BY 6 5 7y ATTORNEY.

United States Patent MEANS COMPENSATING FOR THE TIME DELAY OF ATRANSMISSION LINE Edward C. Grogan, Philadelphia, Pa., assignor toMinneapolis-Honeywell Regulator Company, Minneapolis, Minn, acorporation of Delaware Application December 29, 1955, Serial No.556,319 Claims. (Cl. 137-86) This invention relates to apparatusoperated by an elastic fluid, such as air, which apparatus comprises: ameasuring element, responsive to variations in a measured variable; atransmitter, which converts the variations sensed by the measuringelement into variations in an air pressure; an elongated transmissionline having a small cross section and adapted to conduct the variationsin air pressure; a receiver connected to the transmission line so as toreceive the variations in pressure therefrom; a motor operated byvariations in pressure; and a final control element operated by saidmotor.

It is an object of this invention to provide means for compensating forthe time delays in said transmission line due to distributed resistanceand distributed capacitance in the transmission line.

More specifically, it is an object of this invention to provide areceiver which will feed power back into the transmission line and tothereby compensate for the time delays caused by the transmission line.

A further object of this invention is to provide anelasticfluid-pressure operated circuit in which there is a receiverwhich has bi-lateral action. Bi-lateral action may be defined as anaction having two eifects. One of these is the elfect obtained byfeeding back a portion of the output of the receiver to its input insuch a way as to compensate for the time delays due to the transmissionline leading to the receiver. The other of these elfects is the effectobtained by feeding the remaining portion of the output of the receiverforward to a motor for a final control valve or the like. This forwardfeeding is advanced in phase as compared to the phase of the input, andserves to compensate for any time lags due to the transmission and notalready compensated for by the feedback to the input to the receiver.

It is yet a further object of this invention to provide a circuit inwhich a single pneumatic unit acts as a receiver and performs as twounilateral, ideal, proportional plus rate units, acting in tandem. Thisarrangement therefore serves to minimize considerably the noise problemand to simplify present-day unilateral circuit configurations designedto offer compensation for multi-capacity pneumatic circuits such astubing connected as a transmission line.

The various features of novelty which characterize this invention arepointed out with particularity in the claimsannexed to and forming apart of this specification. For a better understanding of the invention,however, its ad vantages and specific objects obtained with its use,reference should be had to the accompanying drawings and descriptivematter in which is illustrated and described a preferred embodiment ofthe invention.

In the drawings:

Fig. 1 is a diagram showing the circuit connections and the schematicoutlines of the apparatus forming the device;

Fig. 2 is a block diagram forming part of a mathematical analysis;

Fig. 3 is a circuit diagram forming part of this mathematical analysis;and

Fig. 4 is a circuit diagram forming part of this mathematical analysis.

In that embodiment of my invention elected from among others forillustration and in the drawings and description in the specification,my device is shown as follows.

The measuring element M is shown as a flow meter although it may be ameasuring element of many various kinds adapted to sense a variation ina variable quantity or condition. The measuring element M is adapted tomeasure the flow of a fluid through a pipe P. Measuring element Mcomprises an orifice plate 1 having a central opening therethroughcoaxial with the pipe P. Perforated discs 2 and 3 are located onopposite sides of the measuring or orifice plate 1 and serve to conductthe pressure on the opposite sides of the orifice in plate 1 throughpipes 4 and 5 to the transmitter T.

The transmitter T converts the variations in the measured variable intovariations in the pressure of a supply of air.- Transmitter T comprisesa measurement-responsive element formed by two rigid, cup-shaped casings6 and 7 each of which has a wall thereof formed by a flexible diaphragm8 or 9. The centers of diaphragms 8 and 9 are connected together by ashaft or pin 10 to which a flapper 11 is connected so as to movetherewith.

A nozzle 12 cooperates with flapper 11 and is supplied With an elasticfluid, such as compressed air, under pressure from a filtered air supplyF.A.S through a restric- .tion 13 and a pipe 14. A rigid casing 15 isdivided into four chambers 16, 17, 18, and 19 by two diaphragms 20, 21,and by a rigid, perforated wall 22. Diaphragms 20 and 21 are connectedtogether by a hollow tube 23 having a closed, upper end and an open,lower end and an opening 24 through the wall thereof. Air is supplied toinlet chamber 19 by a pipe 28. Ball 26 constitutes an inlet valve incooperation with the rim of a circular opening in wall 22. Spring 27presses inlet valve ball 26 into engagement with the seat formed by therim of the opening through wall 22. Exhaust pipe 23 forms an exhaustvalve by means of the cooperation of the open, lower end of pipe 23 withball 26 so that fluid may pass up through the interior of pipe 23,through hole 24 into chamber 17 whence the air escapes to atmospherethrough hole 25.

The part just described forms a non-bleed high capacity pilot valve orrelay which operates in the following way. When the relay is in itsnormal, steady state, the pressure in chamber 16 is equal to thepressure in chamber 18 (or at some other, preselected ratio thereto),and the inlet valve 26-22 and the exhaust valve 23-26 are both closed.

Now, if the pressure in chamber 16 increases with respect to thepressure in chamber 18, the pressure on diaphragm 20 causes pipe 23 tomove down (as viewed in the drawing) and moves ball 26 away from itscooperating valve seat formed by the rim of the opening through Wall 22.This opens the inlet valve and admits air under pressure from chamber 19into chamber 18. At the same time, the exhaust valve 2326 remains closedbecause of the engagement with the lower end of the exhaust pipe 23 withthe ball 26.

The inlet valve 26-22 remains open until the pressure in the chamber 18is equal to (or at some preselectedratio to) the pressure in chamber 16.When this occurs, the pipe 23 again moves upward (as seen in thedrawing) and the inlet valve 26-22 closes. I

If, on the contrary, the pressure in chamber 16 decreases with relationto the pressure in chamber 18, the inlet valve 26-22 remains closed. Thedecrease in pressure in chamber 16, however, allows pipe 23 to moveupward (as seen in the drawing). This opens the exhaust valve 23--26because the lower end of the pipe 23 moves out of contact with the ball26. Opening the exhaust valve 23--26 connects the chamber 18 to thechamber 17 through the opening 24- so that fluid in chamber" 18 canescape to the atmosphere. When the desired ratio between the pressure inthe chamber 16 and the pressure in the chamber 18 is again restored, thepipe 23 moves downward thereby closing the exhaust valve 2326 and againrestoring the normal, steady state of the relay.

The output of this relay and of transmitter T is connected by pipe 29 tothe input to the transmission line L. Transmission line L comprises atube of approximately A inch internal diameter and may be as much as1,000 feet in length. I

The output end of transmission line L is connected to the input of thereceiver R. Receiver R comprises an input motor formed of a rigid,cup-shaped casing 31 having one wall thereof formed by a flexiblediaphragm 32. A negative feedback motor is formed of a rigid, cupshapedcasing 33 having a flexible diaphragm 34 as a wall thereof. Flexiblediaphragms 32 and 34 are connected by a shaft 35 which supports theflapper 36.

Flapper 36 cooperates with a nozzle 37 to which air under pressure issupplied from a filtered air supply F.A.S. through a restriction 38 anda pipe 39. A rigid casing 40 is divided into four chambers 41, 42, 43,and 44 by two flexible diaphgrams 45 and 46 and a rigid wall 47. Air issupplied to input chamber 44 by pipe 53. Ball 51 is biased by spring 52into engagement with the rim of a circular opening in the center ofrigid wall'47. An exhaust valve is formed by pipe 48, which is closed atits upper end and open at its lower end and connects diaphragms 45 and46. Exhaust pipe 48 has a hole 49 in its side wall communicating withexhaust chamber 42 which communicates with the atmosphere through hole50 in casing 40.

The foregoing elements comprise a second, non-bleed, high capacity pilotvalve or relay which operates in the same way as relay 15-27, describedabove.

The output of this pilot valve or relay is conducted through pipe 54,55, and 56, which form part of a negative feedback circuit including aseries restriction R2 and a pipe 57, which communicates with thenegative feedback motor 33, 34. A bypass connection is formed by abypass restriction R which communicates, at its inlet, with pipe 57 and,at its outlet, with a bypass chamber 60, which has a large capacity ascompared with the capacity of input motor 31, 32 or with the capacity ornegative feedback motor 33, 34.

A positive feedback circuit is formed of pipe 58,'which communicateswithpipes 56 and-55 and with the inlet to a positive feedback restriction RPositive feedback restriction R is connected, at its output side, topipe 30 intermediate the end of transmission line L and the input toinput motor 31, 32.

The output from receiver R is conducted from output chamber 43 by meansof pipes 54 and 61 to the motor V.M., which actuates the final controlelement. Motor V.M. comprises a rigid casing 62 having afiexiblediaphragm 63 forminga wall thereof. A valve 'stem 64 isconnected to the center of diaphragm 63. A spring 65 bears at one endagainst diaphragm 63 and, at its opposite end, against a portion of therigid casing 66.

The'final control element V is shown as avalve controlling the flow ofliquid through a pipe PP which, however, may be the same as pipe P or adiflerent one. The final control element V comprises a rigid casing66-dividedintotwo chambers by a perforated partition 68. A valve plug67, mounted on the end of valve stem 64 cooperates with the hole inpartition 68 so as to open, partially open, or close this hole andtotlier'eby permit or to shut d the flow of fluid throughthe pipe P.

The operation of the apparatus of this invention is as follows. Assumingthe transmitter pilot valve or relay'a'nrl g the receiver pilot valve orrelay to be in their-"steady" state 4. or balanced condition, if avariation in the flow through pipe P is sensed by the measuring elementM, a difference in pressure exists between the discs 2 and 3. Thisdifference in pressure is conducted, by means of pipes 4 and 5, to thechambers formed by the elements 6, 8 and the elements 7, 9,respectively. Any difference between the pressures in each of thesechambers causes shaft 10 to move flapper 11 relative to nozzle 12 in onesense or the other. This movement of flapper 11 varys the flow of fluidthrough nozzle 12 and causes a corresponding. variation of the pressurein the nozzle chamber 16. Since the pressure in nozzle chamber 16 isbalanced against the pressure in outlet chamber 18, a change in pressurein nozzle chamber 16 causes either inlet valve 26 or exhaust valve 23 toopen and to admit or exhaust air from the output chamber 18. Thischanges the air pressure in chamber 18, which change continuesuntil thepressure'in chamber 18 again balances the pressure in nozzle chamber 16.When this balance takes place, the inlet or exhaust valve closes and thepressure ceases to change.

This change in air pressure is transmitted, by means of pipe 29, to theinlet to transmission line L and passes through the transmission line Lwith more or less of a time delay depending upon the resistance andcapacitance of the transmission line L. At the output of the transmission line L, the pipe 30'connects this change in air pressure to theinlet to positive feedback restriction R1 and to the input motor 31, 32.This change in air pressure flows through the restriction R very slowlybecause of the resistance of the restriction R This change in airpressure is applied directly to the input motor 31, 32 and causesimmediate action of the motor to the full extent of the change inpressure. When the device is in balanced condition, the pressure ininput motor 31, 32 is balanced by the pressure in the negative feedbackmotor 33, 34. If a dilference exists between the pressurein inputmotor31, 32 and in the negative feedback motor 33, 34, shaft 35 is moved.This moves flapper 36 relative to nozzle 37 in one sense or the other.Flapper 36 and nozzle 37 constitute first valve of the receiver R. Thisfirst valve 36, 37 controls the pressure in nozzle chamber 41. Thepressure in nozzle chamber 41 is normally balanced by the pressure inoutput chamber 43. If the pressure in nozzle chamber 41 changes, eitherinlet valve '51, or exhaust valve 48 is opened and air is admitted to orexhausted 'fromthe output chamber 43. Thisadmission or exhaust ofair'from the output chamber 43 continues until the pressure in nozzlechamber- 41 again balances the pressure in output chamber 43. Inletvalve 51 and exhaust valve 48 thus constitute the second valve of thereceiver R.

The output air pressure from the pilot valve or relay of receiver 'R isconducted from output chamber 43 through pipes 54 and '61 to the motor62, 63 of the final control element. A change in the pressure in themotor VM causes valve stem 54 to move valve plug 67 relative to theorifice in partition 68 and to thereby vary the flow of fluid throughpipe PP.

Any change in pressure of the output of receiver R is also fed fromoutput chamber 43 through pipes 54, 5S, and 56 to series restriction Rthrough which it passes and thence bymeans of pipe57 to the negativefeedback motor 33, 34. Any change in the pressure in negative feedbackmotor 33, 34 moves shaft 35 until the pressures in the input motor 31,32 and in negative feedback motor 33, 34 are equal. When this occurs,flapper 36 is held at one position relative to nozzle 37 and the deviceis in stable or balanced condition.

The negative fcdback circuit formed of pipes 54, 55, and 56, seriesrestriction R and pipe 57, also includes a bypass restriction R whichcontrols the supply of fluid to a bypass chamber 60, Whose capacity isgreat as compared with'the' capacityof input'motor 31, 32 and thecapacity of negativ. feedback motor 33, 34.

Receiver R also includes'a positive feedback circuit formed of pipe 58which connects, at its input, with pipes 55 and 56 and, at its output,with the positive feedback restriction R The output of positive feedbackrestriction R communicates by means of pipe 59 with pipe 30,intermediate the end of transmission line L and the input to input motor31, 32.

The operation of the pneumatic circuit'described above depends on usingsingle derivative action for effective compensation on distributed lagsthrough pneumatic transmitting elements which behave as multi-capacitysystems (tubing, etc.). This is based upon bi-lateral action across acompensated rate unit (the receiver), which avoids the need for secondderivative response that would ordinarily be required in unilateralarrangements.

The simple pneumatic circuit arrangement shown permits adjustments onrate amplitude and rate time. 'Bilateral action is obtained byresistance coupling between the input and the output of the receiver,which permits the receiver to feed air back into the line in which it isterminated. Since the pressuredeveloped at the output of the receiverleads the transmitted input signal, a regenerative feedback effect willoccur on the pressure signal which is propagated in a line feeding intothe receiver. This action will minimize the effects of line capacitanceand hence speed up signal transmission to the receiver.

The pressure developed at the output 54 of the receiver R can be said tolead in phase the input signal transmitted through the pipe 30 to theinput chamber of the relay R for the following reasons. Anychange of thepressure in the input chamber of the relay R causes a correspondingchange in the output pressure of the relay R in the chamber 43 andconsequently in the pipe 54. This change in pressure is immediatelytransmitted through the pipe 61 to the diaphragm 63 of the valve motorVM.

This change in pressure is also transmitted through the positivefeedback restriction R to the input chamber of the relay R. The changein pressure is felt in the input chamber of the relay R after a timedelay which depends upon the resistance of the restriction R The changein pressure is also fed through pipe 57 to the negative feedback chamber33 of the relay R after a time delay which depends upon the resistanceof the restrictions R and R and upon the capacitance 60. The resistanceof restrictions R R and R and the capacitance of chamber 60 can be soadjusted that the positive feedback restriction R causes a variation inflow to be conducted to the transmission line 30 and to the input motor3132. This variation in flow is large enough to compensate for the timedelay in the transmission of the variations in the pressure passingthrough the supply line L and caused by the resistance and capacitanceof the transmission line L.

The interaction described in the above suggests that the receiver servesas an impedance matching device on a line in which it is terminated.Later herein, it will be shown that the input impedance of the receiverappears as a negative resistance in series with a negative capacitor toground, which act as current sources. In feeding back into amulti-capacity system, the receiver will .of course be limited in itsability to nullify, completely, the effects of distributed lags on thepropagation of a signal transmitted to its input chamber. However, inacting for ward, the receiver transmits a pressure signal (to a valvetion may be analyzed mathematically as follows. Re ferring to Fig. 2,A=Etfective area of diaphragms (assumed equal) on which the input (P andfeedback (P pressures react. k=Gradient (low) of spring loading on thestack. \,,=Slope of flapper-nozzle curve (psi/inch, etc.), G,,=Pressuregain through the pilot valve relay.

The equations of response across the unit may be developed as follows:

Referring to Figure 1: 1) P.= A.G. P.- P.)

where A, k, X and G, are defined above.

For the case where (If G co v 16 Referring again to Figure 2:

P,,-P,, P,,P, (3) I r- R3 assuming:

(a) Ohms law for fluid flow through a pneumatic restriction.

(b) Negligible volume at the junction between R and R3.

From the law for a perfect gas:

where C,=the volumetric capacitance of the volume chamber in which R, isterminated. Substituting Equation 4 into Equation 3:

actuator, etc.), which leads the signal .transmitted to its input motorchamber, and hence will offer additional compensation for distributedresistance-capacitance eifects in a transmission line.

In theory, the bi-lateral action of the device described in the abovewill thereby permit a single compensated proportional plus rate receiverto perform as two unilateral uncompensated proportional plus ratereceivers acting in tandem.

The operation of the pneumatic circuit of this inven- Substituting (6)into the equality (given in (5)),

n( r( 3+ 2) o( 1 3 From Equation 2:-

RAMP;- Thus, substituting Equation 2 into Equation 7:

- t( r( 3+ 2) o( 1- 8 +1) which is the transfer function across acompensated proportional' plus rate unit; where a=rate amplitude ratio,and T =rate time. (Note that adjustments maybe made on both a and TReferring again to Figures 1 and 2 and noting that air flow (19 into theunit is as surned positive: t

Starting with the equation:

J) P.- R P,

and substituting Equation 1 from column 7, into the above;

For Z,-(P)=- %:(P)=input impedance of the unit:

Note that since oc 1, the input impedance of the unit has a negativesign associated with it. -A'circuit with an analogous input impedance asillustrated in Fig. 3. Referring to Figure 3,

Comparing Equations'Z and 3:

(Rs appears to be dead ended).

for a and (RH-R C, for T,

and rearranging terms in Equations 4 and 5:-the settings on R R and Rrequired for desired values of R 0,, and'T, may be determined from thefollowing equations:

a de) 12.0. T, 0. Rio.

Note that required values of R; and C when substituted in the above,will be negative in sign.

Note, also, that adjustments in R R and R will satisfy -a complete rangein values for R1, l, and T in cases where optimum performance requiresthe rate time (T,) of the receiver toexceed the time constant (R C ofits input matching impedance. Otherwise compromises will have to be madein the bi-lateral compensating nature of the receiver v From atheoretical analysis made on the characteristics of tubing when loaded,the following have been determined as optimum settings for R R and R interms of the length of standard in. ID. tubing terminated by thereceiver, when compensating the efiects of distributed transmission lag:

R a.04n p.s.i./in. 3/sec.

tubing length (as feet The above is based on an operating pressure levelof 9 p.s.i.g. and a S-in. volume, terminating thepressure dividercircuit in the negative feedback circuit of the receiver.(i.e., C,=.2lin. ./p.s.i.).

While, in accordance with the provisions of the statutes, :I haveillustrated and described the best forms of the invention now known tome, it will be apparent to those skilledtinthe art that .changesmay bemade in the form of the apparatus disclosed without departing from 9 thespirit of the invention as set forth in the appended claims, and that insome cases certain features of the invention may sometimes be used toadvantage without a corresponding use of other features.

Having now described my invention what I claim as new and desire tosecure by Letters Patent is as follows:

1. A circuit operated by the pressure of an elastic fluid, including, atransmitter responsive to a measuring element which senses variations ina variable, said transmitter controlling the pressure of a first supplyof elastic fluid, an elongated transmission line having a small crosssection and connected to said first supply and causing time delays inthe transmission of the variations in the pressure of said first supplybecause of the resistance and capacitance of said transmission line, anda receiver connected to said transmission line and controlling theoperation of a final control element operated in response to thevariations in the variable, said receiver comprising, an input motorconnected to said transmission line and having a small capacitancecompared to the capacitance of said transmission line and responsive tothe variations in the pressure of said first supply, a first valveactuated by said input motor in one sense and controlling the pressureof a second supply of elastic fluid, a second valve operated by thepressure of said second supply of fluid and controlling the pressure ofa third supply'of fluid adapted for connection to the final controlelement, a negative feedback motor connected to said third supply andactuating said first valve in a sense opposite to that in which saidfirst valve is actuated by said input motor, a manually variable seriesrestriction connected between said third supply and said negativefeedback motor, a manually variable bypass restriction connected at itsinput to the output of said series restriction and to said negativefeedback motor, a bypass chamber having a large capacitance compared tothe capacitance of said negative feedback motor and connected to theoutput of said bypass restriction, and a manually variable positivefeedback restriction connected at its input to said third supply and tothe input of said series restriction and connected at its output to theinput to said input motor, the resistance of each of said restrictionsand the capacitance of said bypass chamber being so adjusted that saidpositive feedback restriction causes a variation in flow to be conductedto said transmission line and to said input motor, said variation inflow being large enough to compensate for the time delay in thetransmission of the variations in the pressure of said first supplybecause of the resistance and capacitance of said transmission line.

2. In a connecting circuit conducting an elastic fluid under pressureand connecting a measuring instrument to a final control element spacedfrom said measuring instrument, means for compensating for the timedelay due to the resistance and capacitance of said circuit andincluding, an input motor actuated by the pressure of an elastic fluid,a valve actuated in one sense by said motor and controlling the pressureof a supply of fluid from a source of fluid under pressure, a negativefeed-back motor actuated by the pressure of said supply of fluid andactuating said valve in the opposite sense to that in which it isactuated by said input motor, a negative feed-back conduit conducting anelastic fluid under pressure and connecting said supply of fluid to saidnegative feed-back motor and including resistance and capacitance, and apositive feed-back circuit conducting elastic fluid under pressure andconnecting said supply of fluid to sa1d mp ut motor and havingresistance and capacitance, sa1d circuits cooperating to compensate forthe time delay due to the resistance and capacitance of said connectingcircuits.

3. In a connecting circuit, means for compensating for the time delaydue to the resistance and capacitance of said circuit and locatedadjacent a final control element and including, an input motor actuatedby the pressure of an elastic fluid, a first valve actuated in one senseby said input motor'andcontrolling the pressure of a first supply offluid from a source of fluid under pressure, a second valve'actuated bythe pressure of said first supply of fluid and controlling the pressureof a second supply of fluid from a source of fluid under pressure, anegative feedback motor actuated by the pressure of an elastic fluid andconnected to said first valve to actuate it in the opposite sense tothat in which it is actuated by said input motor, anegative feed-backconduit conducting an-elastic fluid under pressure and connecting sa1dsecond supplyof fluid and said negative feed-back motorand having acapacitance and resistance, and a posiuve feed-back conduit conductingan elastic fluid under pressure and connecting said second supply offluid and said input motor and having a resistance and capacitance, saidcircuits cooperating to compensate for the time delay due to thecapacitance and resistance of said connecting circuits 4. In aconnecting circuit, means for compensating for the time delay due to theresistance and capacitance of said circuit and located adjacent to afinal control element and including, an input motor actuated by thepressure of an elastic fluid, a valve actuated by said input motor inone sense and controlling the pressure of a supply of fluid from asource of fluid under pressure, a negative feed-back motor actuated bythe pressure of an elastic fluid and actuating said valve in the senseopposite to that in which said valve is actuated by said input motor, anegative feed-back circuit conducting elastic fluid under pressure andconnecting said supply of fluid and said negative feed-back motor andhaving resistance and capacitance, a restriction to the flow of elasticfluid in said negative teed-back circuit connected in series betweensaid supply of fluid and said negative feed-back motor, and apositivetfeed-back circuit conducting an elastic fluid under pressureand connecting said supply of fluid and said input motor and havingresistance and capac ltance, said circuits cooperating to compensate forthe time delay due to the resistance and capacitance of said connectingcircuits.

5. In a connecting circuit connectinga measuring element and a finalcontrol element spaced from said measurlng element, means forcompensating for the time delay caused by the resistance and capacitanceof said circu1t and including, an input motor actuated by the pressureof an elastic fluid, a valve actuated by said input motor in one senseto control the pressure of a supply of fluid from a source of fluidunder pressure, a negative feed-back motor actuated by an elastic fluidunder pressure and connected to said valve to actuate it in a senseopposite to that in which said valve is actuated by said input motor, anegative feed-back circuit conducting an elastic fluid under pressureand connecting said supply of fiuld to said negative feed-back motor andhaving a res1stance and capacitance, a positive feed-back circuitconducting an elastic fluid under pressure and connecting said supply offluid and said input motor, and a restriction to the flow of fluidconnected in said positive feed back motor between said supply of fluidand said input motor, said circuits cooperating to compensate for timedelay due to the resistance and capacitance of said connecting circuit.

6. In a connecting circuit conducting an elastic fluid under pressure,means for compensating for the time delays due to the resistance andcapacitance of said circuit and located adjacent a final control elementand includmg, an input motor actuated by an elastic fluid underpressure, a valve actuated in one sense by said input motor to controlthe pressure of a supply of fluid from a source of fluid under pressure,a negative feed-back motor actuated by an elastic fluid under pressureand connected to said valve so as to actuate it in a sense opposite tothat in which said valve is actuated by said input motor, a negativefeed-back circuit conducting an elastic fluid un- 11 de p e su and cnnecting said' vpp y o fl id and sa negative feed-back motor, arestriction to the flow of fluid connected in series-between the saidsupply of fluid and said negative feed-back motor, a restriction to theflow of fluid connected in said negative feed-back circuit in parallelwith said negative feed-back motor, a closed chamber having a greatercapacity for fluid than said negative feed-back motor and connected tothe output side of said bypass restriction, and a positive feed-backconduit conducting an elastic fluid under pressure and connecting saidsupply of fluid to said input motor and having a resistance andcapacitance, said conduits coperating to compensate for time delay dueto the resistance and capacitance of said connecting circuit.

7. In a connecting circuit, means for compensating for the time delaydue to the resistance and capacitance of said circuit and locatedadjacent a final-control element and including, an input motor actuatedbythe pressure of an elastic fluid, avalve actuated in one sense by saidinput motor to control the pressure of a supply of fluid from a sourceof fluid under pressure, a negative feedback motor actuated by' thepressure of an elastic fluid and connected to said valve so as toactuate it in a sense opposite to that in which said valve is actuatedby said input motor, a negative feed-back circuit conducting an elasticfluid under pressure and connecting said supply of fluid and saidnegative feed-back motor, a restriction to the flow of fluid connectedin series between said supply of fluid and said negative feed-backmotor, a restriction to the flow of fluid connected in said negativefeedback circuit in parallel to said negative feed-back motor, a closedchamber having a greater capacity for fluid than said negative feed-backmotor and connected to the output side of said by-pass restriction, apositive feed-back circuit conducting an elastic fluid under pressureand connecting said supply of fluid and said input motor and having aresistance and capacitance, and a restriction to the flow of fluidconnected in said positive feed-back circuit between the input to saidinput motor and the input to said negative feed-back circuit, saidconduits cooperating to compensate for the time delay due to theresistance and capacitance of said connecting circuit.

8. In a connecting circuit connecting a measuring element to a finalcontrol element, an elongated transmission line conducting elastic fluidunder pressure, means for compensating for the time delay is due to theresistance and capacitance of said transmission line and including, aninput motor actuated by the pressure of an elastic fluid and connectedto that end, of said transmission line adjacent to final controlelement, a valve connected to said input motor so as to be actuatedthereby in one sense to control the pressure of a supply of fluid from asource of fluid under pressure, a negative feed-backtmotor actuated bythe pressure of an elastic fluid and connected to said valve to actuatesaid valve in a sense opposite to that in which said valve is actuatedby said input motor, a negative feed-backcircuit conducting an elasticfluid under pressure and connected to said supply of fluid and to saidnegative feed-back motor and having a resistance and capacitance, and apositive feed-back conduit conducting elastic fluid under pressure andconnecting said supplygof fluid and said input motor and having aresistance and capacitance, said circuits cooperating to compensate forthe time delay due to the resistance and capacitance of said connectingcircuit.

9. A connecting circuit conducting an elastic fluid under pressure andconnecting a measuring element to a final control element and including,a measurement-responsive element, a transmitter valve actuated by saidmeasurement-responsive element and varying the pressure of a firstsupply of fluid from a source of fluid under pressure, an elongatedtransmission line connected to said first supply of fluid, an inputmotor actuated by the pressure of an elastic fluid and connected to theother end of said transmission line and located adjacent to said finalcontrol element, a receiver valve actuated in one sens-e by said inputmotor and controlling the pressure of a second supply of fluid from asource of fluid under pressure, a negative feed-back motor actuated bythe pressure of an elastic fluid and connected to said receiver valveand actuating it in the sense opposite to the sense in which saidreceiver valve is actuated by said input motor, a negative feedbackcircuit conducting elastic fluid under pressure and connecting saidsecond supply of fluid to said negative feed-back motor and having aresistance and capacitance, and a positive feed-back circuit conductingelastic fluid under pressure and connecting said second supply of fluidto said input motor and having a resistance and capacitance, saidcircuits cooperating to compensate for the time delay due to theresistance and capacitance of said connecting circuit.

10. In a connecting circuit conducting an elastic fluid under pressureand connecting a measuring element to a final control element, means forcompensating for the time delay due to the resistance and capacitance ofsaid circuit and including, an elongated transmission line con ductingan elastic fluid at a pressure which is variable in response to adeviation in a measured variable, an input motor connected to one end ofsaid transmission line and adjacent to said final control element andresponsive to variations in said measured variable pressure, a valvemechanically connected to said input motor and operable thereby tochange the pressure of an elastic fluid, a nega tive feed-back motorresponsive to the variations in the pressure of said fluid andmechanically connected to said valve to operate it in the opposite senseto that in which it is moved in response to said deviation, a seriesrestriction connected between said first fluid under pressure and saidnegative feed-back motor, a bypass restriction connected at one end tothe output of said series restriction and to said negative feed-backmotor, a bypass chamber connected to the opposite side of said bypassrestriction and containing an elastic fluid under pressure, and a thirdrestriction connected at its input to said elastic fluid under pressureand to the output of said series restriction and connected at output tosaid input motor and manually adjustable and connected so as to conducta variation in the pressure of the elastic fluid to said input motor,which variation is of a suitable size to compensate for the time delayin the transmission of said measured variable to said input motor.

References Cited in the file of this patent UNITED STATES PATENTSFarrington July 24, 1956

